(1) Field of the Invention
This invention relates to a pneumatic tire suitable for running on snow and ice roads, and more particularly to a pneumatic tire suitable for running on snow and ice roads which has improved tire performances by increasing the tread-gripping force of the tread portion on snow or ice road surface.
(2) Description of the Prior Art
There have heretofore been proposed various pneumatic tires suitable for running on snow and ice roads, but their tire performances are not necessarily satisfactory and further improvement thereof is still demanded. In this connection, investigations have hitherto been made on the materials of the tread portion in order to increase the tread-gripping force on snow or ice road surface. For instance, it is well-known from C. S. Wilkinson; RCT, 27, 255 (1954); F. S. Conant, RCT, 22, 863 (1949) and the like that the hardness of rubber at low temperature is lowered. As is known from the literatures such as W. G. Norich, "The Friction of Polymer on Ice", Journal of the IRI, October, 1972 p.192, Desmond Moore; The Friction of Pneumatic Tires (1975), and the like, the tread-gripping force on the snow or ice road surface is improved by use of butadiene rubber, natural rubber or polyisoprene rubber in the rubber composition of the tread portion. However, according to these methods, the tread-gripping force on wet road surface is disadvantageously lowered. On the other hand, it is known from the literatures such as A. C. Bassi, RCT, 38, 1956; D. Bulgin, G. D. Hubberd, M. H. Walters: Proc. 4th Rubber Tech. Conf. London 193 (1962) and the like that a large amount of styrene-butadiene copolymer rubber, butyl rubber or carbon black is used for mitigating the decrease of the tread-gripping force on the wet road surface. In this case, however, the tire performances on snow and ice roads and rolling resistance are unfavorably lowered.
Further, Japanese Patent Application Publication No. 57-59,256 discloses that the rolling resistance and the wet-braking performance of the tire can be enhanced by adding more than 5 but not more than 30 parts by weight of polyisobutylene-isoprene rubbers containing 1.0-2.0% by weight of chlorine (Cl) or bromine (Br) (halogenated butyl rubber) alone or in admixture to natural rubber (NR) and polybutadiene rubber (BR) without substantially damaging the wear resistance. However, it has been pointed out that such a tire is practically insufficient from the standpoint of the tire performances on snow and ice roads.